Every salt has a solubility equilibrium with its own constant, Ksp. The smaller Ksp is, the less dissolves before equilibrium shuts it down. Drag the slider to change Ksp and watch when precipitation starts.
In pure water, AgCl dissolves until [Ag⁺][Cl⁻] = Ksp. Both ions come from the same source, so their concentrations rise together and equilibrium is reached symmetrically. The molar solubility s gives Ksp = s². Once Q reaches Ksp, no more solid dissolves—the solution is saturated. If you tried to add more AgCl, it would just sit at the bottom undissolved, because the equilibrium position has been reached.
Precipitation happens the instant Q exceeds Ksp, and a common ion suppresses solubility by driving equilibrium left.